1. Technical Field
The invention relates to the field of television signal transmission systems and, in particular, to a method and apparatus for application at the location of a television receiver for improving vertical definition of a television signal by scan conversion.
2. Description of the Relevant Art
There is a growing interest in the transmission of television signals which increase picture definition in both horizontal and vertical dimensions. In the vertical dimension, such a signal may have as many as twice the number of lines in comparison with existing standards. As a result of providing standard resolution, there are adverse effects from providing wide screen displays of a transmitted signal. A viewer of a standard resolution signal may complain of the fuzzy or unclear quality of the displayed image if viewed from a relatively close proximity. The adverse effects are overcome by a higher resolution image but existent transmission systems are not readily adaptable to transmitting a high resolution image.
In sequential scan interlaced systems, a standard resolution 525 line 2:1 interlace NTSC video signal consists of two fields each containing 240 active lines. Lines of every other (odd) field are spacially offset relative to lines of even fields so that all 480 active lines are regularly spaced on the display screen. Neighboring adjacent lines are from alternate (odd or even) fields.
In principle, this line structure can carry a vertical resolution equal to 480 lines for static images. However, a normal interlaced display does not achieve this value. Only 240 lines are displayed in each field and a field is displayed for only 1/60th of a second. The human eye/brain is expected to sum the two fields and perceive all 480 lines which is more easily done for static than dynamic images. The intensity of the first field perceived has decreased to approximately 50% of its initial value by the time the second field arrives 1/60th of a second later. This has two consequences: (i) line structure becomes visible, (ii) vertical frequencies exceeding 240 lines are partially aliased in the display.
The net result is that the perceived vertical resolution of a standard resolution 525 line interlaced display lies somewhere between 240 and 480 lines. The reduction is conventionally described in the art by the "Kell factor" where perceived vertical resolution is equal to 0.66 (the Kell factor).times.480=320 Lines.
For static pictures, the Kell factor may be entirely eliminated and resolution restored to 480 lines by displaying all 480 lines (from both odd and even fields) in each 1/60 second field period. The 240 lines from the previous field transmission are stored in a field store and displayed with the current field transmission. This technique is known as scan conversion. The method, however, only perfectly applies for static images and some compromise is necessary for dealing with dynamic images.
Several manufacturers of television receivers or projectors are using proprietary line doubling techniques including Philips, Hitachi, Sony, Ikegami, etc. Scan conversion line doubling applies equally to component signals (luminance, chrominance) or NTSC signals received from any source. The technique may be applied in the television receiver, a cable television decoder/converter, a satellite signal decoder or practically any device located at or near the receiver location. If applied in a television receiver, the field store can be used for other consumer features such as picture-in-picture or noise reduction.
There remains a requirement in the art, however, to optimize the application of scan converters for dynamic images such that the perceived resolution is improved beyond the level of 320 lines whenever possible with a moving image.